[Qt] Enable sub-pixel layout.
https://bugs.webkit.org/show_bug.cgi?id=113199

Reviewed by Jocelyn Turcotte.

* qmake/mkspecs/features/features.pri:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148594 268f45cc-cd09-0410-ab3c-d52691b4dbfc